This is the old GnB sheet from before 6.2, with the changes to Bloodfest from 90 seconds to 2 minutes this will have changed slightly. You will no longer have any Bloodfest uses outside of your burst so there will be no Bloodfest in your 3 minute burst anymore but instead in your 2 minute, 4 minute, 6 minute, etc burst windows.
While GnB is quite flexible there are certain setups for your burst that you always want to perform.
You can adjust your rotation on a fight per fight basis to gain extra uses of your big hitters, but this is something more advanced and also heavily dependent on your party's killtime so you really shouldn't worry about it until the basic rotation has become muscle memory.
You generally always want to use Gnashing Fang, No Mercy and Bloodfest on cooldown to have them not drift out of party buffs, which is one of the reasons you usually Double Down early to empty your cartridges for Bloodfest.
As for your GCD, 2.43 should be fine. I personally think 2.40 feels the best but with the gear this raid tier the best we can get is unfortunately only 2.42 without losing out on damage. Anything 2.46 or faster should allow you to get the necessary 9 GCDs under No Mercy, anything slower than 2.46 and you will not be able to get those 9 GCDs. There will also be situations where your last Hypervelocity will simply fall out of No Mercy and there is nothing you can do about it.